Cathode electroactive material, production method therefor and secondary cell

ABSTRACT

A cathode electroactive material for use in lithium ion secondary cells, process for producing the material, and lithium ion secondary cells using the cathode electroactive material, wherein the electroactive material predominantly comprises an Li—Mn composite oxide particles with the spinel structure and particles of the electroactive material have an average porosity of 15% or less, the porosity being calculated by employing the following equation:  
     Porosity (%)=( A/B )×100  
     (wherein A represents a total cross-section area of pores included in a cross-section of one secondary particle, and B represents the cross-section area of one secondary particle), a tapping density of 1.9 g/ml or more, a size of crystallites of 400 Å-960 Å, a lattice constant of 8.240 Å or less. The cathode electroactive material of the present invention is formed of particles which are dense and spherical and exhibit excellent packing characteristics to an electrode, and exhibit high initial capacity and capacity retention percentage at high temperature.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a divisional of application Ser. No. 09/785,258, filedFeb. 20, 2001, the disclosure of which is incorporated herein byreference, which claims benefit pursuant to 35 U.S.C. §119(e)(1) of thefiling date of Provisional Application 60/214,794 filed under Jun. 28,2000 pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

[0002] The present invention relates to a cathode electroactive materialfor use in lithium ion secondary cells, a process for producing thematerial, and a lithium ion secondary cell using the cathodeelectroactive material.

BACKGROUND ART

[0003] Lithium manganese composite oxides (hereinafter referred to asLi—Mn composite oxides), which are very safe and are produced fromabundant natural resources, have been of interest for use as a cathodeelectroactive material for lithium ion secondary cells. However, Li—Mncomposite oxides exhibit poor discharge capacity per amount of anelectroactive material as compared with lithium cobalt composite oxides(hereinafter referred to as Li—Co composite oxides). In addition,secondary particles of Li—Mn composite oxide are lightweight and absorba large amount of oil, because the particles contain many pores. Thus,the amount of electroactive material which can be fed into adimensionally limited cell must be restricted, thereby disadvantageouslylowering the electrochemical capacity of a unit cell.

[0004] In recent years, U.S. Pat. No. 5,807,646 (Japanese PatentApplication Laid-Open (kokai) No. 9-86933) has proposed measures tocounter the aforementioned problem. Specifically, a mixture of amanganese compound and a lithium compound is shaped at a pressure of 500kg/cm² or higher, heated, and crushed, to thereby produce an Li—Mncomposite oxide having a tapping density (i.e., apparent density ofpowder in a container which is moved, e.g., vibrated under certainconditions) of 1.7 g/ml or higher. However, the disclosed tappingdensity is at most 1.9 g/ml, which is unsatisfactory.

[0005] The above official gazette also discloses the average particlesize of secondary particles which are formed by aggregating primaryparticles of an Li—Mn composite oxide. However, even when the packingdensity of secondary particles is enhanced through the interactionbetween primary particles, secondary particles are disintegrated duringthe electrode material (paste) preparation step. Thus, controlling theaverage particle size of the secondary particles is not a fundamentalcounter-measure.

[0006] Some methods for producing a spinel-type Li—Mn composite oxidehave already been disclosed. Japanese Patent Application Laid-Open(kokai) No. 9-86933 discloses such a method comprising burning a mixtureof a manganese compound and a lithium compound at a high temperature,e.g., 250° C. to 850° C. Japanese Patent Application Laid-Open (kokai)No. 4-237970 discloses such a method comprising mixing a manganesecompound, a lithium compound, and an oxide of boron which can besubstituted by manganese and burning the resultant mixture at a hightemperature, to thereby produce an Li—Mn—B oxide in which Mn atoms arepartially substituted with B, and the Li—Mn—B oxide serves as a cathodeelectroactive material.

[0007] When the aforementioned materials are burned at high temperaturein air or in an oxygen gas flow, the secondary particles obtainedthrough crushing have a high average porosity (15% or more) and a lowtapping density (1.9 g/ml or less). Thus, thus-obtained cathodeelectroactive materials cannot be charged into an electrode in a largeamount, and thereby, a high discharge capacity cannot be attained.

[0008] Japanese Patent Application Laid-Open (kokai) No. 4-14752discloses a cathode electroactive material employing a manganese oxidewhich is produced by mixing spinel-type lithium manganese oxide andtitanium oxide and sintering the resultant mixture. However,disadvantageously, titanium oxide only reacts with lithium and manganeseat 950° C. to 1000° C. or higher to form a melt, and a tapping densityof 1.60 g/ml can be only attained by adding titanium oxide in an amountas large as 10 mass %.

DISCLOSURE OF INVENTION

[0009] An object of the present invention is to provide a cathodeelectroactive material for us in lithium ion secondary cells, whichelectroactive material has an excellent packing property and exhibits ahigh initial discharge capacity and a low decrease in discharge capacityafter charging and discharging are repeated (hereinafter the property isreferred to as high “capacity retentions”).

[0010] The present inventors have conducted extensive studies, and havesolved the aforementioned problems by successfully densifying particlesof an Li—Mn composite oxide. Specifically, the spinel-type Li—Mncomposite oxide is burned and crushed. Then, a sintering agent is addedto the resultant pulverized particles, and the particles are granulatedand burned.

[0011] Accordingly, the present invention provides a cathodeelectroactive material for use in lithium ion secondary cells, a processfor producing the material, a paste for producing an electrode and acathode electrode for use in lithium ion secondary cells comprising acathode electroactive material, and a lithium ion secondary cell asdescribed below.

[0012] [1] A cathode electroactive material for use in lithium ionsecondary cells, wherein the cathode electroactive materialpredominantly comprises Li—Mn composite oxide particles with the spinelstructure and particles of the electroactive material have an averageporosity of 15% or less, the porosity being expressed by the followingequation:

Porosity (%)=(A/B)×100   (1)

[0013] (wherein A represents a total cross-section area of poresincluded in a cross-section of one secondary particle, and B representsthe cross-section area of one secondary particle).

[0014] [2] A cathode electroactive material for use in lithium ionsecondary cells as described in [1], wherein the average porosity is 10%or less and the average particle size of primary particles is 0.2 μm-3μm.

[0015] [3] A cathode electroactive material for use in lithium ionsecondary cells as described in [1], wherein the tapping density of thecathode electroactive material is 1.9 g/ml or more.

[0016] [4] A cathode electroactive material for use in lithium ionsecondary cells as described in [3], wherein the tapping density of thecathode electroactive material is 2.2 g/ml or more.

[0017] [5] A cathode electroactive material for use in lithium ionsecondary cells as described in [1], wherein the size of crystallitescontained in the cathode electroactive material is 400 Å-960 Å.

[0018] [6] A cathode electroactive material for use in lithium ionsecondary cells as described in [1], wherein the lattice constantdetermined with respect to the electroactive material is 8.240 Å orless.

[0019] [7] A cathode electroactive material for use in lithium ionsecondary cells as described in [1], wherein the electroactive materialis produced by granulating an Li—Mn composite oxide with the spinelstructure serving as a predominant component comprising an oxide whichis molten at 550° C.-900° C.: an element which forms the oxide: acompound comprising the element; an oxide which forms a solid solutionor melts to react with lithium or manganese: an element which forms theoxide: or a compound comprising the element, and sintering the formedgranules.

[0020] [8] A cathode electroactive material for use in lithium ionsecondary cells as described in [7], wherein the oxide which is moltenat 550° C.-900° C.: or the element which forms the oxide: or thecompound comprising the element; or the oxide which forms a solidsolution or melts to react with lithium or manganese: or the elementwhich forms the oxide: the compound comprising the element, is at leastone element selected from the group consisting of Bi, B, W, Mo, and Pb:or a compound comprising the element; a compound comprising B₂O₃ andLiF; or a compound comprising MnF₂ and LiF.

[0021] [9] A process for producing a cathode electroactive material foruse in lithium ion secondary cells predominantly comprising an Li—Mncomposite oxide with the spinel structure, which comprises adding, to apulverized Li—Mn composite oxide with the spinel structure, an oxidewhich is molten at 550° C.-900° C.: an element which forms the oxide: acompound comprising the element: an oxide which forms a solid solutionor melts to react with lithium or manganese: an element which forms theoxide: or a compound comprising the element; and mixing, to thereby formgranules.

[0022] [10] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], which processcomprises sintering the granules in addition to forming granules.

[0023] [11] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], which processcomprises, in addition to forming granules, sintering the granules byelevating the temperature of the granules from asintering-shrinkage-initiating temperature to a temperature higher thanthe sintering-shrinkage-initiating temperature by at least 100° C. at arate of at least 100° C./minute; successively maintaining the elevatedtemperature for one minute-10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute.

[0024] [12] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [11], wherein thesintering is carried out by use of a rotary kiln.

[0025] [13] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [10], wherein atleast one element selected from the group comprising of Bi, B, W, Mo,and Pb: the compound comprising the element; a compound comprising B₂O₃and LiF; or a compound comprising MnF₂ and LiF is molten on the surfacesof particles of Li—Mn composite oxide so as to carry out the abovedescribed sintering process.

[0026] [14] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], whereinpulverized Li—Mn composite oxide with the spinel structure has anaverage particle size of 5 μm or less.

[0027] [15] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], whereinpulverized Li—Mn composite oxide with the spinel structure has anaverage particle size of 3 μm or less.

[0028] [16] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], whereingranulation process is carried out through spray granulation, agitationgranulation, compressive granulation, or fluidization granulation.

[0029] [17] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [9], wherein at leastone organic compound selected from the group consisting of acrylicresin, an isobutylene-maleic anhydride copolymer, poly(vinyl alcohol),poly(ethylene glycol), polyvinylpyrrolidene, hydroxypropyl cellulose,methyl cellulose, cornstarch, gelatin, and lignin is employed as agranulation aid during granulation process.

[0030] [18] A process for producing a cathode electroactive material foruse in lithium ion secondary cells as described in [17], which processcomprises binder removal process in air or in an oxygen-containingenvironment at 300° C. to 550° C.

[0031] [19] A cathode electroactive material for use in lithium ionsecondary cells which is produced through a process as described in anyone of [9] to [18].

[0032] [20] A paste for producing an electrode comprising a cathodeelectroactive material for use in lithium ion secondary cells as claimedin any one of claims [1] to [8].

[0033] [21] A cathode electrode for a lithium ion secondary cell, whichthe electrode comprises a cathode electroactive material for use inlithium ion secondary cells as described in any of [1] to [8] or [19].

[0034] [22] A lithium ion secondary cell equipped with a cathodeelectrode for a lithium ion secondary cell as described in [21].

[0035] [23] A lithium ion secondary cell as described in [22], which isformed into a coin-shaped cell, a coil cell, a cylinder-shaped cell, abox-shaped cell, or a lamination cell.

BRIEF DESCRIPTION OF DRAWINGS

[0036]FIG. 1 shows an example (Example 14) of photographic images(scanning electron microscope, ×15,000) of the cathode electroactivematerial according to the present invention, which was granulated,burned, and size-adjusted.

[0037]FIG. 2 shows an example (Example 14) of particle size distributionof the cathode electroactive material according to the presentinvention, which was granulated, burned, and size-adjusted.

DETAILED DESCRIPTION OF INVENTION

[0038] The present invention will next be described in detail.

[0039] The present invention relates to a spinel-type Li—Mn compositeoxide, in which secondary particles of the electroactive material have aporosity of 15% or less. The porosity is considerably reduced ascompared with the electroactive material of a conventional electrode.The present invention also relates to a spinel-type Li—Mn compositeoxide, in which secondary particles of the oxide have an averageporosity of 10% or less. The electrochemical cycle characteristics ofthe oxide are more excellent than those of a conventional Li—Mncomposite oxide.

[0040] The cathode electroactive material of the present inventioncomprising a spinel-type lithium-magnesium (Li—Mn) composite oxidecollectively refers to compounds represented by LiMn₂O₄,Li_(1+x)Mn_(2−x)O₄ (0<x<0.2), or Li_(1+x)Mn_(2−x−y)M_(y)O₄ (0<x<0.2,0<y<0.4) in which Mn is partially substituted by at least one element(represented by M in the formula) selected from the group consisting ofchromium, cobalt, aluminum, nickel, iron, and magnesium.

[0041] The cathode electroactive material of the present invention foruse in lithium ion secondary cells, in which the electroactive materialpredominantly comprises a spinel-type Li—Mn composite oxide andsecondary particles of the electroactive material have an averageporosity of 15% or less, the porosity of one secondary particle beingcalculated by employing the following equation:

Porosity (%)=(A/B)×100   (1)

[0042] (wherein A represents a total cross-section area of poresincluded in a cross-section of one secondary particle, and B representsthe cross-section area of one secondary particle).

[0043] In aforementioned Li—Mn composite oxide, the average porosity ofthe aforementioned cathode electroactive material is preferably 10% orless, and the average particle size of primary particles is 0.2 μm-3 μm.

[0044] In order to attain a tapping density of the cathode electroactivematerial in excess of 1.9 g/ml, the average porosity of the secondaryparticles is required to be 15% or less, preferably 13% or less, morepreferably 10% or less.

[0045] When sintering is carried out at a high temperature for a longperiod of time in a typical process for producing a composite oxide soas to reduce the average porosity of secondary particles to be as low aspossible through sintering, primary particles are grown to largeparticles as sintering proceeds. Employment of the thus-producedmaterial as a cathode electroactive material for a cell results in adecrease in the capacity retention of the cell. Thus, cells fabricatedfrom the material have poor cell performance.

[0046] The present inventors have conducted extensive studies on amethod for sintering with suppressing particle growth, and have foundthat sintering with suppressing particle growth can be brought about byelevating the temperature of granules from asintering-shrinkage-initiating temperature to a temperature higher thanthe sintering-shrinkage-initiating temperature (as measured throughthermo-mechanical analysis) by at least 100° C. at a rate of at least100° C./minute; successively maintaining the elevated temperature forone minute to 10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute.

[0047] The term “sintering-shrinkage-initiating temperature” hereinrefers to a shrinkage-initiating temperature measured throughthermo-mechanical analysis. The aforementioned maintained temperature isrequired to be higher than the sintering-shrinkage-initiatingtemperature by at least 100° C. When the maintained temperature iselevated by less than 100° C., the sintering-shrinkage rate is small,leading to a longer sintering time. As a result, particles are grown toa primary particles size of more than 0.5 μm.

[0048] The time for maintaining the elevated temperature is one minuteor longer and 10 minutes or shorter so as to attain a primary particlesize of 0.2 μm or more and 0.5 μm or less and excellent cellcharacteristics. The temperature is higher than thesintering-shrinkage-initiating temperature by at least 100° C. duringthe aforementioned sintering step. To maintain the temperature for lessthan one minute is not sufficient for thermal conduction, and particleshaving a primary particle size as small as less than 0.2 μm and poorcrystallinity will be produced, thereby lowering the initial dischargecapacity. When the time is in excess of 10 minutes, particles continueto grow after sintering is completed, thereby elevating the primaryparticle size and lowering the capacity retention percentage.Accordingly, in the present invention, the time for maintaining thetemperature is preferably 2-8 minutes, more preferably 2-5 minutes.

[0049] Temperature elevating and lowering rates between thesintering-initiating temperature and th temperature for maintaining theelevated temperature are set to be at least 100° C./minute for thefollowing reasons: The time during which the temperature is maintainedin the temperature region where particles are grown is made as short aspossible, for allowing only sintering to proceed, preventing growth ofparticles.

[0050] In order to attain a tapping density of the cathode electroactivematerial in excess of 2.2 g/ml, the average porosity of the secondaryparticles is required to be 10% or less, preferably 7% or less, morepreferably 5% or less.

[0051] In the present invention, the size of crystallites comprised inthe aforementioned cathode electroactive material is preferably 400Å-960 Å. When the size is less than 400 Å, crystallinity isinsufficient, thereby lowering the initial discharge capacity of thecell and the capacity retention percentage, whereas when the size is inexcess of 960 Å, the capacity retention percentage drasticallydecreases. More specifically, the size is preferably 500 Å-920 Å, morepreferably 700 Å-920 Å.

[0052] The lattice constant determined with respect to the cathodeelectroactive material of the present invention comprising a spinel-typeLi—Mn composite oxide is preferably 8.240 Å or less. When the latticeconstant is in excess of 8.240 Å, the capacity retention percentagedrastically decreases. Accordingly, the lattice constant is preferably8.235 Å or less, more preferably 8.233 Å or less.

[0053] The cathode electroactive material of the present inventionpredominantly comprising a spinel-type Li—Mn composite oxide is formedof dense granulated particles which are prepared by crushing a burnedspinel-type Li—Mn composite oxide; adding a sintering agent (granulationaccelerator) to the resultant pulverized particles (i.e., secondaryparticles which are formed by aggregating primary particles andpreferably have an average particle size of 0.5 μm or less); and burningto granulate. The term “dense granulated particles” herein refers toparticles in which no or few pores are contained between primaryparticles of the oxide. The cathode electroactive material of thepresent invention is formed of the aforementioned dense substance, andis formed by employment of a sintering agent mentioned in below.

[0054] Hereinafter the process for producing the cathode electroactivematerial of the present invention will be described.

[0055] The process for producing a spinel-type Li—Mn composite oxidecomprises burning a mixture containing a manganese compound, a lithiumcompound, and an optional compound containing a hetero-element which canbe substituted by manganese, in air or an oxygen gas flow at 300°C.-850° C. for at least one hour.

[0056] No particular limitation is imposed on the crystallinity of thespinel-type Li—Mn composite oxide, and an unreacted lithium compound ormanganese compound may remain in the composite oxide. When thespinel-type Li—Mn composite oxide has a high crystallinity, the latticeconstant thereof is not particularly limited. However, employment of aspinel-type Li—Mn composite oxide having a lattice constant of 8.240 Åor less as a cathode electroactive material prevents decrease incapacity retention percentage.

[0057] No particularly limitation is imposed on the raw material forproducing the spinel-type Li—Mn composite oxide, and known manganesecompounds such as manganese dioxide, dimanganese trioxide, trimanganesetetraoxide, hydrated manganese oxide, manganese carbonate, and manganesenitrate; and lithium compounds such as lithium hydroxide, lithiumcarbonate, and lithium nitrate are employed.

[0058] Preferably, manganese carbonate is suitable for theaforementioned manganese compound in that manganese carbonate readilyreacts with a lithium compound at low temperature. An Li—Mn oxide ofcathode electroactive material obtained from manganese carbonate impartsexcellent properties to cells. In order to produce manganese-substitutedLi—Mn—M (hetero-element) composite oxide represented byLi_(1+x)Mn_(2−x−y)O₄, at least one element selected from the groupconsisting of chromium, cobalt, aluminum, nickel, iron, and magneseiumis added to the aforementioned manganese compound and lithium compoundserving as the raw materials. Any M-containing compound (hetero-element)can be used so long as the compound forms the aforementioned oxidethrough thermal reaction, and the M-containing compound may be added tothe manganese compound and lithium compound during thermal reaction.

[0059] No particular limitation is imposed on the method for crushingand pulverizing secondary particles of the aforementioned spinel-typeLi—Mn composite oxide, and known crushers and pulverizers can beemployed. Examples include a medium-stirring type pulverizer, a ballmill, a paint shaker, a jet-mill, and a roller mill. Crushing andpulverizing may be performed in a dry manner or a wet manner. Noparticular limitation is imposed on the solvent employed in thewet-manner crushing and pulverizing, and solvents such as water andalcohol may be employed.

[0060] Particle size of the crushed and pulverized spinel-type Li—Mncomposite oxide is important in view of acceleration of sintering. Theparticle size measured by means of a laser particle size distributionmeasurement apparatus is preferably 5 μm or less. More preferably, nocoarse particles having a size 5 μm or more is contained, and containedparticles have an average particle size of 2 μm or less. Still morepreferably, no coarse particles having a size more than 3 μm iscontained, and contained particles have an average particle size of 1.5μm or less. The particle size is further preferably 0.5 μm or less, yetfurther preferably 0.3 μm or less, particularly preferably 0.2 μm orless.

[0061] No particular limitation is imposed on the method for mixing asintering agent with the crushed and pulverized spinel-type Li—Mncomposite oxide. For example, mixing may be carried out by use of amedium-stirring type crushing machine, a ball mill, a paint shaker, or amixer. Mixing may be performed in a dry manner or a wet manner. Thesintering agent may be added to the Li—Mn composite during crushing andpulverizing the oxide.

[0062] The sintering agent is not particularly limited, so long as itenables sintering of crushed and pulverized particles of the Li—Mncomposite oxide for granulation of the particles. The sintering agent ispreferably a compound which melts at 900° C. or lower. For example, thecompound may be an oxide which melts at 550-900° C. or a precursor whichmay be converted into the oxide; or an oxide which forms a solidsolution with lithium or manganese or reacts with lithium or manganeseto form a melt, or a compound which may be converted into the oxide.

[0063] The sintering agent, for example, may be a compound comprising anelement such as Bi, B, W, Mo, or Pb. Such compounds may be employed incombination. The sintering agent may be a compound comprising B₂O₃ andLiF, or a compound comprising MnF₂ and LiF. The sintering agent is morepreferably a compound comprising Bi, B, or W, since such a compoundgreatly exerts sintering (sintering-shrinkage) effect.

[0064] Examples of Bi compounds include bismuth trioxide, bismuthnitrate, bismuth benzoate, bismuth hydroxyacetate, bismuth oxycarbonate,bismuth citrate, and bismuth hydroxide. Examples of B compounds includeboron sesquioxide, boron carbide, boron nitrid, and boric acid. Examplesof W compounds include tungsten dioxide and tungsten trioxide.

[0065] The amount of a sintering agent which is added to the compositeoxide is 0.0001-0.05 mol (as reduced to metallic element in the agent)on the basis of 1 mol of Mn in the Li—Mn composite oxide. When theamount is less than 0.0001 mol, the sintering agent exerts no sintering(sintering-shrinkage) effect, whereas when the amount is in excess of0.05 mol, the initial capacity of the electroactive material comprisingthe composite oxide becomes low. The amount to be added is preferably0.005-0.03 mol.

[0066] The sintering agent may be used in the form of powder, or may bedissolved in a solvent and used in the form of solution. When thesintering agent is employed in the form of powder, the agent preferablyhas an average particle size of 50 μm or less, more preferably 10 μm orless, much more preferably 3 μm or less. The sintering agent ispreferably added to the crushed composite oxide particles beforegranulation and sintering of the particles. Alternatively, aftergranulation of the particles, resultant granules may be impregnated withthe sintering agent at a temperature at which the agent melts, and thensintering may be carried out.

[0067] A sintering agent often remains after the sintering step in thecathode material for use in cells. For example, the aforementionedsintering agent used in the producing process of the present inventionis detected by analysis to remain in the cathode electroactive material.

[0068] A method for granulation will next be described.

[0069] Granulation may be carried out by use of the aforementionedsintering agent through spray granulation, flow granulation, compressiongranulation, or stirring granulation. The granulation may be carried outin combination with medium-flow drying or medium-vibration drying.

[0070] In the present invention, no particular limitation is imposed onthe method for granulation so long as dense secondary particles(including granulated particles) are formed. Stirring granulation andcompression granulation are particularly preferred in consideration ofproduction of secondary particles having a high density. Spraygranulation is also particularly preferred in consideration ofproduction of granules having a round shape. Examples of stirringgranulation apparatuses include a vertical granulator (product ofPaurec) and Spartanryuzer (product of Fuji Paudal). Examples ofcompression granulation apparatuses include a roller compactor (model:MRCP-200, product of Kurimoto Tekko). Examples of spray granulationapparatuses include a mobile-minor-type spray dryer (product ofAshizawaniro Atomizer).

[0071] No particular limitation is imposed on the size of secondaryparticles to be granulated. When the average size of the granulatedsecondary particles is very large, the particles may be lightly crushedand pulverized immediately after granulation or after sintering of theparticles, and then subjected to size-regulation such as classification,to thereby obtain the granules of desired size. Typically, secondaryparticles having an average particle size of 10-20 μm are preferred.

[0072] In order to enhance granulation efficiency, an organicgranulation aid may be added.

[0073] Examples of these granulation aids include an acrylic resin, anisobutylene-maleic anhydride copolymer, poly(vinyl alcohol),poly(ethylene glycol), polyvinylpyrrolidone, hydroxypropyl cellulose,methyl cellulose, cornstarch, gelatin, and lignin.

[0074] Although the granulation aid may be added in the form of powder,the granulation aid is preferably added by spraying it dissolved inwater or an organic solvent such as alcohol in view of granulationefficiency. The granulation aid is added preferably in an amount of fiveparts by weight or less on the basis of 100 parts by weight of a mixtureof the sintering agent and the spinel-type Li—Mn composite oxide, morepreferably two parts by weight or less.

[0075] A method for sintering the granulated secondary particles willnext be described.

[0076] The binder contained in the granulated secondary particles isremoved at 300-550° C. for 10 minutes or more in air or in anoxygen-containing gas flow. The amount of residual carbon in thebinder-free granules is preferably 0.1% or less.

[0077] In order to proceed sintering with suppressing growth ofparticles, binder-removed granules are fired in air or anoxygen-containing gas flow at 550° C. to 900° C. for one minute orlonger. Under these conditions, the sintering agent is maintained moltenon Li—Mn composite oxide particles, thereby densifying secondaryparticles through sintering.

[0078] In the present invention, binder-free granulated particles isburned in air or an oxygen-containing gas flow under the followingconditions so as to suppress growth of particles and proceed sintering.Specifically, the procedure includes elevating the temperature from asintering-shrinkage-initiating temperature measured throughthermo-mechanical analysis to a temperature higher than thesintering-shrinkage-initiating temperature by at least 100° C. at a rateof at least 100° C./minute; successively maintaining the elevatedtemperature for one minute-10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute,to thereby attain sintering and densify the secondary particles.Temperature elevation and lowering between ambient temperature and thesintering-shrinkage-initiating temperature may be 10° C./min or less ashas been conventionally employed.

[0079] Even when the aforementioned organic granulation aid is notemployed, sintering of the granules may be carried out in air or in anoxygen-gas-flow atmosphere in a manner as described above, to therebyproduce dense secondary particles.

[0080] The cathode electroactive material of the present invention andthe cathode electroactive material produced through the method forproducing the same according to the present invention are formed into acathode electrode of lithium ion secondary cells, and performance of thecell are evaluated through methods similar to those employed for aconventional Li—Mn composite oxide.

[0081] Hereinafter, example methods for employing the cathodeelectroactive material of the present invention as a material of acathode electrode in the non-aqueous secondary cell will next bedescribed.

[0082] The cathode material is produced through the following procedure:kneading the cathode electroactive material, a conductivity-impartingagent such as carbon black or graphite, and a binder such aspolyvinylidenefluoride dissolved in a solvent (e.g.,N-methylpyrrolidone) in predetermined proportions; applying theresultant electrode paste to a current-collecting material; drying; andpressing the paste-applied material by use of a roll press or a similarapparatus. The current-collecting material may be a known metalliccurrent-collecting material such as aluminum, stainless steel, ortitanium.

[0083] In the non-aqueous secondary cell according to the presentinvention, an electrolytic salt contained in an electrolytic solutionmay be a known fluorine-containing lithium salt. For example, LiPF₆,LiBF₄, LiN(CF₃SO₂)₂, LiAsF₆, LiCF₃SO₃, or LiC₄F₉SO₃ may be employed. Theelectrolytic solution employed in the non-aqueous secondary cell isproduced by dissolving at least one species of the aforementioned knownfluorine-containing lithium salts in a non-aqueous electrolyticsolution. The aforementioned non-aqueous solvent for the non-aqueouselectrolytic solution is not particularly limited, so long as thesolvent is chemically or electrochemically stable and aprotic.

[0084] Examples of such solvents include carbonic acid esters such asdimethyl carbonate, propylene carbonate, ethylene carbonate, methylethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate,methyl butyl carbonate, diethyl carbonate, ethyl propyl carbonate,diisopropyl carbonate, dibutyl carbonate, 1,2-butylene carbonate, ethylisopropyl carbonate, and ethyl butyl carbonate; oligoethers such astriethylene glycol methyl ether and tetraethylene glycol dimethyl ether;aliphatic esters such as methyl propionate and methyl formate; aromaticnitriles such as benzonitrile and tolunitrile; amides such asdimethylformamide; sulfoxides such as dimethyl sulfoxide; lactones suchas γ-butyrolactone; sulfur compounds such as sulforane;N-vinylpyrrolidone; N-methylpyrrolidone; and phosphoric acid esters. Ofthese, carbonic acid esters, aliphatic esters, or ethers are preferred.

[0085] In the non-aqueous secondary cell of the present invention, thematerial of an anode electrode is not particularly limited, so long asit can reversibly occlude or release lithium ions. For example, thematerial may be lithium metal, lithium alloy, carbon material (includinggraphite), or metal-chalcogen.

[0086] A method for evaluation of electrode characteristics will next bedescribed.

[0087] The cathode electroactive material, Vulcan XC-72 (product ofCabot Corp.) serving as a conductive material, and anethylenetetrafluoride resin serving as a binder are mixed in proportionsby weight of 50:34:16, and the resultant mixture is swollen with tolueneover 12 hours. The swollen mixture is applied onto a current-collectingmaterial comprising aluminum expanded metal, and shaped at a pressure of2 t/cm², and then toluene is dried, to thereby produce a cathodeelectrode. An anode electrode is produced from lithium foil.

[0088] Propylene carbonate and dimethyl carbonate are mixed at a ratioby volume of 1:2, and LiPF₆ is dissolved in the resultant mixture in aconcentration of 1 mol/liter, to thereby produce an electrolyticsolution. A separator formed of polypropylene is employed. In order toprevent micro short circuit due to formation of dendrite in the anodeelectrode, for example, silica fibrous filter paper QR-100 (product ofAdvantec Toyo Co.) serving as a reinforcing material is employed incombination. A 2016-type coin-shaped cell is fabricated from the cathodeelectrode, the anode electrode, the electrolytic solution, theseparator, and the reinforcing material. The thus-fabricated cell issubjected to charging and discharging test of 500 cycles in a thermostatof 60° C. Measurement conditions are as follows:constant-current-constant-voltage charging and constant-currentdischarging; charging or discharging rate 1C (charging time: 2.5 hours);and scanning voltage 3.1-4.3 V.

BEST MODE FOR CARRYING OUT THE INVENTION

[0089] The present invention is described below by referring to Examplesand Comparative Examples, however, the present invention should not beconstrued as being limited thereto.

[0090] The characteristics of the cathode electroactive material asshown in examples below and Tables 1-3 was evaluated according to thefollowing procedures.

[0091] 1) Average Particle Size and Specific Surface Area

[0092] The powder was dispersed in a 0.2% aqueous solution of Demol P(Kao Corporation) by the application of ultrasound, and the particlesize distribution was measured by means of a laser particle sizedistribution measuring apparatus (GRANULOMETER, Model HR 850, product ofCILAS).

[0093] 2) Tapping Density

[0094] The tapping density was measured by vertically vibrating 2000times at an amplitude of 8 mm using a tapping machine (type KRS-409,Kuramochi Kagaku Kiki Seisakusho).

[0095] 3) Porosity

[0096] The cathode electroactive material was embedded in resin bymixing and hardening the cathode electroactive material andthermosetting resin, and the resin solid was cut by means of amicrotome. The cut surface was mirror-polished, and the thus-polishedsurface was observed by Scanning Electron Microscope (SEM). Thecross-section area of one secondary particle (B) calculated from theobtained SEM photographic image and the total cross-section area of allpores (A) included in the cross-section area of one secondary particlewere determined by means of an image-analyzer. The porosity (C) (%) ofone secondary particle was calculated according to the following formulato determine the average porosity from the average of 50 secondaryparticles selected at random:

C(%)=(A/B)×100

[0097] 4) Crystallite Size

[0098] The crystallite size was determined by employing Sherrer'sformula from the peak corresponding to a (111) face as measured underthe following conditions through X-ray diffractometry.

[0099] On the assumption that the crystallites are cubic and constant insize, broadening of the diffraction peak depending on the size ofcrystallite was calculated on the basis of the half-width.Monocrystalline silicone was pulverized using a sample mill made oftungsten carbide and sieved to a size of 44 μm or less. The apparatusconstant calibration curve was determined by the employment the sievedpowder as an external standard.

[0100] [Measurement Apparatus and Method]

[0101] The measurement apparatus employed to analyze the size of thecrystallites was a Rad-type goniometer (Rigaku Denki) (measurement mode:continuous), and the analysis software employed was RINT 2000 Series(application software, Rigaku Denki).

[0102] The measurement conditions were as follows: X-ray; CuKα ray,output power; 50 kV, 180 mA; slit widths (3 points); 1/2°, 1/2°, and0.15 mm, scanning method; 2θ/θ, scanning rate: 1°/min; measuring range(2θ); 17-20°, and step; 0.004°. The measurement accuracy for thecrystallite size fell within ±30 Å.

[0103] 5) Lattice Constant

[0104] The lattice constant was obtained through a method described byJ. B. Nelson and D. P. Riley (Proc. Phys. Soc., 57, 160 (1945)).

[0105] 6) Specific Surface Area

[0106] Specific Surface Area was obtained according to BET method.

[0107] 7) Shape of Granulated Particles

[0108] Granules of cathode electroactive materials were photographed bySEM. Through analysis of the obtained images, roundness(roundness=4π[area/(circumference)²]) and aspect ratio (aspectratio=absolute maximum length of needle/diagonal width) of secondaryparticles were obtained. The average values of 200 secondary particleswere measured for each sample.

EXAMPLE 1

[0109] Manganese carbonate having a specific surface area of 22 m²/g(C2-10; product of Chuo Denki Kogyo) and lithium carbonate (3N, productof Honjo Chemical) were mixed together at an element ratio of 0.51(Li/Mn) using a ball mill. The resultant mixture was heated from roomtemperature to 650° C. at a rate of 200° C./hour in air. The temperaturewas maintained for four hours, to thereby obtain an Li—Mn compositeoxide. X-ray diffraction analysis apparatus (XRD) revealed that inaddition to Li—Mn composite oxide, a trace amount of dimanganesetrioxide was also contained in the synthesized product. The averageparticle size of the product measured by means of a laser particle sizedistribution measuring apparatus was 10 μm, and the specific surfacearea thereof was 7.7 m²/g.

[0110] The obtained Li—Mn composite oxide having spinal structure wasdispersed in ethanol solvent and pulverized with a wet ball mill suchthat the average particle size became 0.5 μm. Measurement revealed thatparticles having a particle size of 3 μm or more were not contained, andthat the specific surface area of the particles was 27.8 m²/g. Thepowder was mixed with bismuth oxide having an average particle size of 2μm so as to attain a Bi/Mn element ratio of 0.0026. The resultantmixture was granulated with Spartanryuzer RMO-6H (Fuji Paudal).

[0111] An aqueous solution of polyvinyl alcohol (1.5 parts by weight),serving as a granulation aid, was added to the mixed powder (100 partsby weight) of the Li—Mn composite oxide and bismuth oxide, and was thenfollowed by granulation for 16 minutes. The obtained granulatedsubstance was lightly crushed and then pulverized in a mixer, to therebyobtain powder having an average particle size of 15 μm as measured by apneumatic classifier. The tapping density of the size-adjusted granuleswas 1.65 g/ml.

[0112] The resultant granules were left to stand under atmosphericcondition, in air, at 500° C. for two hours, to thereby remove thebinder from the granules (i.e., to decompose polyvinyl alcohol in thegranules). Thereafter, the resultant granules were sintered at 750° C.in air to 750° C. at a rate of 200° C./hour, and then maintained at75020 C. for 20 hours, to hereby produce a cathode electroactivematerial. Inductively coupled plasma emission spectroscopy (ICP-AES)confirmed that elemental Bi derived from the bismuth oxide was presentin the electroactive material in an amount corresponding to the amountof bismuth oxide employed.

[0113] The average porosity of the obtained cathode electroactivematerial was found to be 11.2%. The tapping density and crystallite sizeof the material were 1.96 g/ml and 880 Å, respectively. The latticeconstant measured with respect to the cathode electroactive material was8.233 Å.

[0114] Using the thus-obtained cathode electroactive material, acoin-shaped cell was fabricated as follows. The cathode electroactivematerial, carbon black serving as a conductor, andpolyvinylidenefluoride in N-methyl-2-pyrrolidone were kneaded inproportions by weight of 80:10:10. The resultant substance was appliedto aluminum foil and then pressed, to thereby obtain a cathodeelectrode. Lithium foil having a predetermined thickness was used as ananode electrode. Propylene carbonate and dimethyl carbonate were mixedat a volume ratio of 1:2. LIPF₆ was dissolved in the obtained mixedliquid at a concentration of 1 mol/liter. The resultant solution wasused as an electrolyte. Using the thus-obtained cathode electrode, anodeelectrode, and electrolyte as well as a polypropylene separator and aglass filter, a 2016-type coil-shaped cell was fabricated.

[0115] The fabricated cell was tested at 60° C. over subjection to 100charge-discharge cycles, each performed at a charge-discharge rate of 1Cand within a voltage range of 3.0 V to 4.2 V. Table 1 shows the initialdischarge capacity and the capacity retention percentage (%) as measuredafter the 100-cycles test with the other results of measurement.

EXAMPLE 2

[0116] The procedure of Example 1 was repeated, except thatelectrolytically produced manganese dioxide serving as a manganesesource, to thereby synthesize an Li—Mn composite oxide. As is similar toExample 1, porosity, tapping density, crystallite size, and latticeconstant of the secondary particles, and electrode performance wereevaluated. The results are shown in Table 1.

EXAMPLE 3

[0117] Manganese carbonate, lithium carbonate, and aluminum hydroxidewere mixed together at proportions by element of 1.02:1.967:0.013(Li/Mn/Al) using a ball mill. The resultant mixture was heated from roomtemperature to 650° C. at a rate of 200° C./hour in air. The temperaturewas maintained at 650° C. for four hours, to thereby synthesize an Li—Mncomposite oxide. XRD revealed that in addition to Li—Mn composite oxide,a trace amount of dimanganese trioxide was also contained in thesynthesized product. The average particle size of the product measuredby means of a laser particle size distribution measuring apparatus was10 μm.

[0118] The produced Li—Mn composite oxide was crushed to particleshaving an average particle size of 0.5 μm. Boron oxide was added to theparticles so as to adjust the element ratio (B/Mn) to 0.0208, and themixture was granulated. Subsequently, the procedure of Example 1 wasrepeated, except that binder-free granulates were burned at 750° C. for0.5 hour. The results of evaluation are shown in Table 1.

EXAMPLE 4

[0119] The procedure of Example 3 was repeated, except that the elementratio (B/Mn) was adjusted to 0.009 and binder-free granules were burnedat 760° C. for 0.5 hour. The results of evaluation are shown in Table 1.

EXAMPLE 5

[0120] The procedure of Example 3 was repeated, except that the elementratio (B/Mn) was adjusted to 0.006 and binder-free granules were burnedat 770° C. for 0.5 hour. The results of valuation are shown in Table 1.

EXAMPLE 6

[0121] The procedure of Example 1 was repeated, except that binder-freegranules were burned at 760° C. for 20 hours. The results of evaluationare shown in Table 1.

EXAMPLE 7

[0122] The procedure of Example 1 was repeated, except that tungstentrioxide was used instead of bismuth oxide; tungsten trioxide was addedin an element ratio (W/Mn) of 0.0208; and binder-free granules wereburned at 750° C. for 20 hours. The results of evaluation are shown inTable 1.

EXAMPLE 8

[0123] The Li—Mn composite oxide which had been synthesized in Example 1was further heated from room temperature to 750° C. in air at a heatingrate of 200° C./hour, and the thus-heated oxide was maintained at 750°C. for 20 hours, to thereby crystallize. The procedure of Example 1 wasrepeated, except that crystallized Li—Mn composite oxide was used, boronoxide was used instead of bismuth oxide; boron oxide was added in anelement ratio (B/Mn) of 0.0208; and binder-free granules were burned at750° C. for 0.5 hour. The results of evaluation are shown in Table 1.

EXAMPLE 9

[0124] The procedure of Example 3 was repeated, except that Li—Mncomposite oxide particles having an average particle size of 3.5 μm anda specific surface area of 10 m²/g were employed as ungranulatedparticles. The results of evaluation are shown in Table 1.

EXAMPLE 10

[0125] The procedure of Example 3 was repeated, except that manganesecarbonate, lithium carbonate, and aluminum hydroxide were mixed togetherat proportions by element of 1.03:1.967:0.013 (Li/Mn/Al) using a ballmill, to thereby synthesize an Li—Mn composite oxide. The results ofevaluation are shown in Table 1. TABLE 1 60° C. Cell Mol Burning Cathodeelectroactive material performance ratio of conditions Specific Capacityadded after Tapping surface Initial retention sintering binder- densityarea Crystallite Lattice Round- Aspect capacity after 100 No agent freePorosity % g/ml m²/g size Å constant Å ness ratio mAh/g cycles % Ex. 1Bi/Mn 750° C. × 20 hr 11.2 1.96 1.8 880 8.233 0.76 1.31 129 84 0.0026Ex. 2 Bi/Mn 750° C. × 20 hr 12.0 1.93 1.8 890 8.234 0.75 1.33 118 780.0026 Ex. 3 Bi/Mn 750° C. × 0.5 hr 6.5 2.16 1.2 780 8.232 0.78 1.28 12785 0.0208 Ex. 4 Bi/Mn 760° C. × 0.5 hr 2.3 2.33 1.0 910 8.231 0.78 1.29125 83 0.0090 Ex. 5 Bi/Mn 770° C. × 0.5 hr 1.8 2.35 0.9 930 8.230 0.771.28 126 81 0.0060 Ex. 6 Bi/Mn 760° C. × 20 hr 9.1 2.05 1.2 910 8.2310.78 1.29 115 87 0.0026 Ex. 7 Bi/Mn 750° C. × 20 hr 6.1 2.18 1.1 8008.239 0.78 1.28 124 76 0.0208 Ex. 8 Bi/Mn 750° C. × 0.5 hr 9.8 2.02 1.5820 8.240 0.76 1.30 128 80 0.0208 Ex. 9 Bi/Mn 750° C. × 0.5 hr 8.5 2.071.3 750 8.233 0.79 1.31 126 85 0.0208 Ex. 10 Bi/Mn 750° C. × 0.5 hr 6.32.15 1.2 750 8.228 0.76 1.29 116 89 0.0208

EXAMPLE 11

[0126] The procedure of Example 1 was repeated, except that binder-freegranules were burned at 830° C. for 20 hours. The results of evaluationare shown in Table 2.

EXAMPLE 12

[0127] The procedure of Example 3 was repeated, except that manganesecarbonate, lithium carbonate, and aluminum hydroxide were mixed togetherat proportions by element of 0.99:1.967:0.013 (Li/Mn/Al) using a ballmill, to thereby synthesize an Li—Mn composite oxide. The results ofevaluation are shown in Table 2.

EXAMPLE 13

[0128] The procedure of Example 3 was repeated, except that the averageparticle size of granulated particles was adjusted to 65 μm. The resultsof evaluation are shown in Table 2.

EXAMPLE 14

[0129] The procedure of Example 1 was repeated, except that the elementratio (Bi/Mn) was adjusted to 0.0020. The results of evaluation areshown in Table 2. The obtained cathode electroactive material, which wasgranulated, burned, and size-adjusted, was observed by a scanningelectron microscope (SEM) (×15,000). As shown in FIG. 1, the particleshave be n found to be spherical. The particle size distribution of theseparticles is shown in FIG. 2.

COMPARATIVE EXAMPLE 1

[0130] The procedure of Example 1 was repeated, except thatnon-granulated particles of the Li—Mn composite oxide before undergoinggranulation had an average particle size of 6.0 μm. The results ofevaluation are shown in Table 2.

COMPARATIVE EXAMPLE 2

[0131] Lithium carbonate and electrolytically synthesized manganesedioxide having an average particle size of 20 μm were mixed together atan element ratio (Li/Mn) of 0.51 using a ball mill, and the mixture washeated to 760° C. at a heating rate of 100° C./hour and the heatedmixture was maintained at 760° C. for 24 hours, to thereby synthesize acathode electroactive material. The thus-obtained cathode electroactivematerial was evaluated in a manner similar to that employed inExample 1. The results are shown in Table 2.

COMPARATIVE EXAMPLE 3

[0132] The procedure of Example 1 was repeated, except that granulationwas performed without adding a sintering agent. The results ofevaluation are shown in Table 2.

COMPARATIVE EXAMPLE 4

[0133] The procedure of Example 3 was repeated, except that granuleswere burned at 750° C. for 20 hours. The results of evaluation are shownin Table 2. TABLE 2 60° C. Cell performance Mol Burning Cathodeelectroactive material Capacity ratio of conditions Specific retentionsintering after Tapping surface Initial after agent binder- density areaCrystallite Lattice Round- Aspect capacity 100 No added free Porosity %g/ml m²/g size Å constant Å ness ratio mAh/g cycles % Ex. 11 Bi/Mn 830°C. × 20 hr 2.0 2.34 0.8 960 8.235 0.74 1.35 127 71 0.0026 Ex. 12 Bi/Mn750° C. × 0.5 hr 6.4 2.14 1.4 770 8.243 0.73 1.34 131 74 0.0208 Ex. 13Bi/Mn 750° C. × 0.5 hr 6.6 2.48 1.2 790 8.233 0.84 1.25 128 85 0.0208Ex. 14 Bi/Mn 750° C. × 20 hr 15.0 1.83 2.0 850 8.235 0.74 1.32 131 840.0200 Comp. Bi/Mn 750° C. × 20 hr 16.6 1.74 2.5 930 8.233 0.72 1.33 12077 Ex. 1 0.0026 Comp. — 760° C. × 24 hr 16.0 1.71 6.8 600 8.239 0.661.45 110 73 Ex. 2 Comp. — 750° C. × 20 hr 19.6 1.62 4.8 580 8.232 0.711.32 125 83 Ex. 3 Comp. Bi/Mn 750° C. × 20 hr 5.4 2.20 0.3 >1000 8.2340.69 1.39 122 54 EX. 4 0.0208

EXAMPLE 15

[0134] Manganese carbonate, lithium carbonate, and aluminum hydroxidewere mixed together at proportions by element of 1.02:1.967:0.013(Li/Mn/Al) using a ball mill. The resultant mixture was heated from roomtemperature to 650° C. at a rate of 200° C./hour in air. The temperaturewas maintained at 650° C. for four hours, to thereby synthesize an Li—Mncomposite oxide. XRD revealed that in addition to Li—Mn composite oxide,a trace amount of dimanganese trioxide was also contained in thesynthesized product. The average particle size of the product measuredby means of a laser particle size distribution measuring apparatus was10 μm.

[0135] Boron oxide was added to the obtained Li—Mn composite oxide so asto attain an element ratio (B/Mn) of 0.0208. The resultant mixture wasdispersed in ethanol solvent and pulverized with a wet ball mill suchthat the average particle size became 0.3 μm. The resultant mixture wasgranulated with Spartanryuzer RMO-6H (Fuji Paudal).

[0136] An aqueous solution of polyvinyl alcohol (1.5 parts by mass),serving as a granulation aid, was added to the mixed powder (100 partsby mass) of the Li—Mn composite oxide and boron oxide, and was thenfollowed by granulation for 16 minutes. The obtained granulatedsubstance was lightly crushed and then pulverized in a mixer, to therebyobtain powder having an average particle size of 15 μm as measured by apneumatic classifier. The tapping density of the size-adjusted granuleswas 1.60 g/ml.

[0137] The resultant granules were left to stand under atmosphericcondition at 500° C. for two hours, to thereby remove binders of thegranules (i.e., to decompose polyvinyl alcohol in the granules).Thermo-mechanical analysis of the binder-free granulates revealed thatsintering-shrinkage-initiating temperature of the granulates was 660° C.

[0138] Subsequently, binder-free granulates were sintered by use of arotary kiln under the following conditions.

[0139] The temperature of the uniform-heat zone of the rotary kiln wasadjusted to 780° C. Feeding rate of granules, and rotation speed andinclination of the rotary kiln were tuned such that the binder-freegranules pass through the uniform-heat zone for three minutes. Timerequired for transferring the binder-free granules from the inlet to theuniform-heat zone and that required for transferring the granules fromthe uniform-heat zone to the outlet of the kiln were 6.3 minutes,respectively.

[0140] The average porosity of the obtained cathode electroactivematerial was found to be 2.1%. The longest particle size of each of 500primary particles was on an SEM image, and the average particle size wasfound to be 0.40 μm.

[0141] Using the thus-obtained cathode electroactive material, acoin-shaped cell was fabricated as in the same way as in Example 1.

[0142] The fabricated cell was tested at 60° C. over 100charge-discharge cycles, each performed at a charge-discharge rate of 1Cand within a voltage range of 3.0 V to 4.2 V.

[0143] Table 3 shows the initial discharge capacity and the capacityretention percentage (%) as measured after the 100-cycle test.

EXAMPLE 16

[0144] The temperature of the uniform-heat zone of the rotary kiln wasadjusted to 780° C. The procedure of Example 15 was repeated, exceptthat feeding rate of granules and rotation speed and inclination of therotary kiln were tuned such that the debindered granules pass throughthe uniform-heat zone for nine minutes. The results of evaluation areshown in Table 3.

EXAMPLE 17

[0145] Manganese carbonate, lithium carbonate, andvapor-phase-synthesized alumina were mixed together at proportions byelement of 1.02:1.967:0.013 (Li/Mn/Al) using a ball mill. The resultantmixture was heated from room temperature to 650° C. at a rate of 200°C./hour in air. The temperature was maintained at 650° C. for fourhours, to thereby synthesize an Li—Mn composite oxide. XRD revealed thatin addition to Li—Mn composite oxide, a trace amount of dimanganesetrioxide was also contained in the synthesized product. The averageparticle size of the product measured by means of a laser particle sizedistribution measuring apparatus was 10 μm.

[0146] Boron oxide was added to the obtained Li—Mn composite oxide so asto attain an element ratio (B/Mn) of 0.0104. The resultant mixture wasdispersed in ion-exchange water, and pulverized with a medium-stirringmicro-pulverizer such that the average particle size became 0.18 μm.Granulation aid (Isobam 104 Kuraray Co., Ltd.) was added to theresultant slurry in an amount of 1.5 mass % based on the Li—Mn compositeoxide, and dry-granulation was carried out by use of a disk-rotatingspray-drier. The granulated substance was found to be sphericalparticles having an average particle size of 18.3 μm and a tappingdensity of 1.54 g/ml.

[0147] The thus-prepared granulates were allowed to stand underatmospheric condition at 500° C. for two hours for removal of binders.The binder-free granules were sintered by use of a rotary kiln underconditions similar to those employed in Example 15.

[0148] The obtained cathode electroactive material was found to have anaverage porosity of 1.7%, an average particle size of 0.27 μm, a tappingdensity of 2.40 g/ml, and a specific surface area (BET) of 0.8 m²/g. Acoin-shaped cell was fabricated from the cathode electroactive materialin a manner similar to that employed in Example 15. The cell performanceis shown in Table 3.

EXAMPLE 18

[0149] The procedure of Example 15 was repeated, except that thetemperature of the uniform-heat zone of a rotary kiln was adjusted to850° C. The results of evaluation are shown in Table 3.

EXAMPLE 19

[0150] The procedure of Example 17 was repeated, except that thetemperature of the uniform-heat zone of a rotary kiln was adjusted to850° C. The results of evaluation are shown in Table 3.

COMPARATIVE EXAMPLE 5

[0151] The procedure of Example 15 was repeated, except that binder-freegranulates were heated from 650° C. to 750° C. at a rate of 10°C./minute, maintained at 750° C. for 0.5 hour, sintered, and cooled to650° C. at 10° C./minute. The obtained cathode electroactive materialwas evaluated in a manner similar to that employed in Example 15. Theresults of evaluation are shown in Table 3.

COMPARATIVE EXAMPLE 6

[0152] The procedure of Comparative Example 5 was repeated, except thatsintering was carried out at 750° C. for 20 hours. The results ofevaluation are shown in Table 3.

COMPARATIVE EXAMPLE 7

[0153] The temperature of the uniform-heat zone of the rotary kiln wasadjusted to 780° C. The procedure of Example 15 was repeated, exceptthat feeding rate of granules, and rotation speed and inclination of therotary kiln were tuned such that the binder-free granules pass throughthe uniform-heat zone for 0.5 minute, and time required for transferringthe binder-free granules from the inlet to the uniform-heat zone andthat required for transferring the granules from the uniform-heat zoneto the outlet of the kiln were 1.5 minutes, respectively. The results ofevaluation are shown in Table 3. TABLE 3 Cathode electroactive material60° C. Cell Av. performance primary Specific Capacity Tapping particlesurface Initial retention Sintering density size area Lattice Aspectcapacity after 100 No conditions Porosity % g/ml μm m²/g constant ÅRoundness ratio mAh/g cycles % Ex. 15 780° C. × 3 min 2.1 2.35 0.40 0.88.233 0.77 1.31 127 90 120° C./min Rotary kiln Ex. 16 780° C. × 9 min1.6 2.44 0.50 0.4 8.236 0.75 1.29 128 87 120° C./min Rotary kiln Ex. 17780° C. × 3 min 1.7 2.40 0.28 0.8 8.235 0.99 1.02 127 91 120° C./minRotary kiln Ex. 18 850° C. × 9 min 1.4 2.51 2.66 0.3 8.237 0.73 1.29 12886 120° C./min Rotary kiln Ex. 19 850° C. × 9 min 1.3 2.52 2.41 0.28.237 0.75 1.29 128 87 120° C./min Rotary kiln Comp. 750° C. × 0.5 hr6.5 2.14 0.55 1.2 8.232 0.78 1.28 127 85 Ex. 5  10° C./min Box furnaceComp. 750° C. × 20 hr 2.3 2.33 0.84 0.3 8.235 0.78 1.29 125 70 Ex. 6 10° C./min Box furnace Comp. 780° C. × 0.5 min 13.1 1.92 0.19 2.1 8.2330.77 1.28 118 81 Ex. 7 120° C./min Rotary kiln

[0154] Measurement and Analysis of Shape of Granulated Particles

[0155] Through analysis of the secondary particles produced in Examples1 to 19 and Comparative Examples 1 to 7 shown in Tables 1 to 3,roundness (roundness=4π[area/(circumference)²]) and aspect ratio (aspectratio=absolute maximum length of needle/diagonal width) of secondaryparticles were obtained. The cathode electroactive materials accordingto the present invention were found to have a roundness of 0.7 or more,and an aspect ratio of 1.35 or less.

[0156] Industrial Applicability

[0157] The cathode electroactive material of the present invention isdefinitely different from conventional electroactive material comprisingsecondary particles formed on the basis of cohesive force, since thecathode electroactive material of the present invention is producedthrough granulation and sintering. The material is formed of particleswhich are dense and spherical and exhibit excellent packingcharacteristics to an electrode, as compared with cathode electroactivematerial obtained through a conventional process for producing the same.In addition, the cathode electroactive material serves as a materialwhich enhances initial discharge capacity and capacity retentionpercentage of secondary cells even at high temperature.

[0158] The process of the present invention for producing a cathodeelectroactive material includes adding a sintering agent forming a meltat high temperature to th Li—Mn composite oxide, to thereby densifysecondary particles. The process of the present invention is alsoadvantageous as compared with conventional processes in that excellentcell performance can be attained even when crystallites have a sizewhich is detrimental to initial capacity and cycling characteristics.During densification of secondary particles, there is a problem thatprimary particles are grown to a particle size more than 0.5 μm, therebylowering initial capacity and cycling characteristics. The presentinvention can solve the problem by adding a sintering agent forming amelt at high temperature to the Li—Mn composite oxide, and provides acathode electroactive material having high packing characteristics andexcellent cell performance.

[0159] The lithium ion secondary cell of the present invention employs acathode electroactive material having an excellent packing property,accordingly, exhibits high initial capacity and capacity retentionpercentage at high temperature.

What is claimed is:
 1. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells predominantly comprisingan Li—Mn composite oxide with the spinel structure, which comprisesadding, to a pulverized Li—Mn composite oxide with the spinel structure,an oxide which is molten at 550° C.-900° C.: an element which forms theoxide: a compound comprising the element: an oxide which forms a solidsolution or melts to react with lithium or manganese: an element whichforms the oxide: or a compound comprising the element; and mixing, tothereby form granules.
 2. A process for producing a cathodeelectroactive material for use in lithium ion secondary cells as claimedin claim 1, which process comprises sintering the granules in additionto forming granules.
 3. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells as claimed in claim 1,which process comprises, in addition to forming granules, sintering thegranules by elevating the temperature of the granules from asintering-shrinkage-initiating temperature to a temperature higher thanthe sintering-shrinkage-initiating temperature by at least 100° C. at arate of at least 100° C./minute; successively maintaining the elevatedtemperature for one minute-10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute.4. A process for producing a cathode electroactive material for use inlithium ion secondary cells as claimed in claim 3, wherein the sinteringis carried out by use of a rotary kiln.
 5. A process for producing acathode electroactive material for use in lithium ion secondary cells asclaimed in claim 2, wherein at least one element selected from the groupcomprising of Bi, B, W, Mo, and Pb: the compound comprising the element;a compound comprising B₂O₃ and LiF; or a compound comprising MnF₂ andLiF is molten on the surfaces of particles of Li—Mn composite oxide soas to carry out the above described sintering process.
 6. A process forproducing a cathode electroactive material for use in lithium ionsecondary cells as claimed in claim 1, wherein pulverized Li—Mncomposite oxide with the spinel structure has an average particle sizeof 5 μm or less.
 7. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells as claimed in claim 1,wherein pulverized Li—Mn composite oxide with the spinel structure hasan average particle size of 3 μm or less.
 8. A process for producing acathode electroactive material for use in lithium ion secondary cells asclaimed in claim 1, wherein granulation process is carried out throughspray granulation, agitation granulation, compressive granulation, orfluidization granulation.
 9. A process for producing a cathodeelectroactive material for use in lithium ion secondary cells as claimedin claim 1, wherein at least one organic compound selected from thegroup consisting of acrylic resin, an isobutylene-maleic anhydridecopolymer, poly(vinyl alcohol), poly(ethylene glycol),polyvinylpyrrolidene, hydroxypropyl cellulose, methyl cellulose,cornstarch, gelatin, and lignin is employed as a granulation aid duringgranulation process.
 10. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells as claimed in claim 9,which process comprises binder removal process in air or in anoxygen-containing environment at 300° C. to 550° C.
 11. A cathodeelectroactive material for use in lithium ion secondary cells which isproduced through a process as claimed in claim
 1. 12. A paste forproducing an electrode comprising a cathode electroactive material foruse in lithium ion secondary cells, wherein the cathode electrodematerial predominately comprises Li—Mn composite oxide particles with aspinel structure and particles of the electroactive material have anaverage porosity of 15% or less, the porosity being expressed by thefollowing equation: Porosity (%)=(A/B)×100   (1) (wherein A represents atotal cross-section area of pores included in a cross-section of onesecondary particle, and B represents the cross-section area of onesecondary particle).
 13. A cathode electrode for a lithium ion secondarycell, in which the electrode comprises a cathode electroactive materialfor use in lithium ion secondary cells, wherein the cathode electrodematerial predominately comprises Li—Mn composite oxide particles with aspinel structure and particles of the electroactive material have anaverage porosity of 15% or less, the porosity being expressed by thefollowing equation: Porosity (%)=(A/B)×100   (1) (wherein A represents atotal cross-section area of pores included in a cross-section of onesecondary particle, and B represents the cross-section area of onesecondary particle).
 14. A lithium ion secondary cell equipped with acathode electrode for a lithium ion secondary cell as claimed in claim13.
 15. A lithium ion secondary cell as claimed in claim 14, which isformed into a coin-shaped cell, a coil cell, a cylinder-shaped cell, abox-shaped cell, or a lamination cell.